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Factory modification to a Syncrowave 250, troubleshooting help?

I have a 1989 Syncrowave and the current adjustment knob is "compressed" so that 0 amps starts at a about 70 A dial setting. It uses the older type PC1 board. The high end is OK so that I get 300 A at the max setting. Adjusting the pots that set max and min current will not trim this out.

It looks like the op-amp that measures the shunt had some components replaced around it with a tacked on capacitor and spade lug to ground. Someone added a third wire to the shunt also. Does anyone know if this could have been a factory mod that isn't documented? --Not sure why anyone would do this. The foot pedal in remote works fine also (0 current to max current fully depressed, no matter the dial setting).

I agree. If that is the cause, the potentiometer could be replaced with the correct linear taper potentiometer. It's likely that the original potentiometer went bad and was replaced with the wrong style. After all, a potentiometer is a potentiometer, isn't it? Not it's not.

Thanks everyone, I measured the voltage on the pot wiper and it is linear from zero to the max setting. But when I plotted the current output as a function of the pot position, its like the first 1/3 turn does nothing. Also, the relay, CR1, doesn't actuate at all until I get past the 1/3 turn.

Thanks everyone, I measured the voltage on the pot wiper and it is linear from zero to the max setting. But when I plotted the current output as a function of the pot position, its like the first 1/3 turn does nothing. Also, the relay, CR1, doesn't actuate at all until I get past the 1/3 turn.

Voltage tests won't tell much if the potentiometer is still in the circuit. What is important is the resistance of the pot. when disconnected from the circuit.

If the first third of the pot's movement does little or nothing, then it is by definition non-linear, i.e., Audio taper. This is further shown by your statement that the relay doesn't move until past the first 1/3 turn. This proves that the resistance is greater in that position than a similar linear pot. would be.

I'm betting with those who say that the wrong pot. was installed. I would remove the pot and test it for resistance and, if non-linear, replace it with a liner pot of the correct wattage.